The present invention relates to self-regulating heaters and methods for manufacturing such heaters. The self-regulating heaters of the present invention have particular utility for soldering applications but are suitable for a variety of other applications in which regulated localized heating is desirable.
Specific soldering applications in which the heater of the present invention is useful are disclosed in the following co-pending U.S. Pat. Applications: S.N. 07/277,116 filed Nov. 29, 1988 filed by McKee et al and entitled "Self Regulating Temperature Heater With Thermally Conductive Extensions"; S.N. 07/277,361 filed Nov. 19, 1988 filed by McKee et al and entitled "Self Regulating Temperature Heater Carrier Strip"; and SN 07/277,362 filed by McKee et al and entitled "Surface Mount Technology Breakaway Self Regulating Temperature Heater"; all filed concurrently herewith and owned by the same assignee as the present application. The disclosures in all of the above patent applications are expressly incorporated herein in their entireties.
The present invention makes use of a relatively new automatic self-regulating heater technology disclosed in U.S. Pat. Nos. 4,256,945 (Carter et al), 4,623,401 (Derbyshire et al), 4,659,912 (Derbyshire), 4,695,713 (Krumme), 4,701,587 (Carter et al), 4,714,814 (Krumme) and 4,745,264 (Carter). The disclosures in these patents are expressly incorporated herein by reference. A heater constructed in accordance with this technology, hereinafter referred to as a self-regulating heater, employs an electrical conductor of copper, copper alloy or other material of low electrical resistivity, negligible magnetic permeability and high thermal conductivity. A surface layer of thermally-conductive magnetic material is disposed on all or part of the surface of the conductor, the surface layer material typically being iron, nickel or nickel-iron alloy, or the like, having a much higher electrical resistance and magnetic permeability than the conductor material The thickness of the surface layer is approximately one skin depth, based on the frequency of the energizing current and the permeability and resistance of the surface layer. A constant amplitude high frequency alternating energizing current is passed through the heater and, as a result of the skin effect phenomenon, is initially concentrated in one skin depth corresponding to the thickness of the magnetic surface layer. When the temperature at any point along the heater reaches the Curie temperature of the magnetic material, the magnetic permeability of the material at that point decreases dramatically, thereby significantly increasing the skin depth so that the current density profile expands into the non-magnetic conductor of low resistivity. The overall result is a lower resistance and lesser heat dissipation. If thermal sinks or loads are placed in contact with the heater at different locations along the heater length, thermal energy is transferred to the loads at those locations with the result that the temperature does not rise to the Curie temperature as quickly at those locations as it does in the non-loaded locations. The constant amplitude current remains concentrated in the higher resistance surface layer at the loaded locations which dissipate considerably more resistive heating energy than is dissipated in the non-load locations where the current is distributed in the low resistance conductor.
In order to effect multiple soldering operations simultaneously it is convenient to configure the self-regulating heater as a substrate of electrically conductive material on which the magnetic surface layer is deposited or otherwise disposed Unless otherwise constrained, the energizing current passing through the heater is distributed according to the skin effect phenomenon at all of the heater surface portions, not merely the surface portions clad with the magnetic surface layer. Thus, unless all of the surface portions of the substrate conductor are clad with the magnetic surface layer, the effectiveness and efficiency of the self-regulating feature of the heater are significantly diminished. In the aforementioned U.S. Patent Application SN 277,116, for example, there is described a heater embodiment in which both the top and bottom surfaces of the substrate conductor are clad with the magnetic surface material, it being recognized that the surface area corresponding to the thickness dimension of the substrate is so small as to have negligible effect on the overall current distribution in the substrate if permitted to remain unclad with the magnetic material. This configuration, also useful for some soldering applications, has the disadvantage of a relatively low impedance. More particularly, since the two surface layers are effectively connected in parallel, the overall impedance of the heater is lower than would be the case if the current were some how constrained to flow along only one surface. The importance of the low impedance relates to the necessity for matching the impedance of the load (i.e., the heater) to the impedance of the energizing current source in order to maximize power transfer to the load. Since the impedance of the source is typically on the order of fifty ohms, and the impedance of the heater is typically on the order of one ohm, an impedance matching circuit capable of a fifty-to-one matching ratio is required. This ratio is difficult enough to attain without further exacerbation brought about by a reduced heater impedance. Moreover, if the load impedance is lowered, the required constant amplitude for the energizing current must be significantly increased to achieve the desired heating level.
Current can be constrained to flow primarily in the magnetic surface layer on a single surface of the heater by establishing an electric field oriented perpendicular to that surface. This can be achieved by providing a return bus for the energizing current, the bus being positioned such that one surface of the bus is spaced from the magnetic surface layer by a thin layer of insulation; the heater is then connected in series with the return bus across the energizing source. The direction of current flow through the heater at any instant of time is longitudinally opposite the direction of current flow through the return bus. A resulting electric field is developed across the insulation layer and acts to constrain the current through the heater to flow primarily in the magnetic surface layer The return bus and insulation may be part of the tooling assembly employed during a soldering operation and must be both physically and electrically connected to the heater each time a soldering operation is performed.
It is desirable to provide a self-regulating heater which can be clad on only one surface without sacrificing its effectiveness as a self-regulating heater, yet which is simple to employ in a soldering operation. It is also desirable for the heater to be simple and inexpensive to fabricate. Further, the heater impedance that is useful in generating thermal energy should be maximized in order to: maximize thermal energy generated by the heater; minimize the amplitude of the energizing current; and reduce the impedance matching ratio between the energizing source and the heater.